1. Field of the Invention
The present invention relates to information storage devices, and more particularly to an improved servopattern for implementation in a storage device wherein errors introduced into the servopattern are reduced.
2. Discussion of Related Art
Increased levels of storage capacity in storage devices such as removable disk-drives, hard disk drives, and tape drives are a direct result of higher track densities possible with voice-coil and other types of servo positioners as well as the ability to read and write narrower tracks by using, for example, magnetoresistive (MR) head technology. Head positioning is accurately controlled using positional information stored on the disk itself known as embedded servo architectures.
Conventional servopatterns in an “embedded servo” disk drive architecture as shown in FIG. 1a comprise short servo bursts of constant frequency signal that precede data regions of the track and are used to align a head with respect to the track center. Although FIG. 1a shows 8 servo bursts, disk drives can have 200 or more servo bursts on a track, or one roughly every 2 degrees.
A typical servopattern used in DASD products, commonly referred to as half track servo patterns, deploys a servo pattern track pitch equal to one half of the product track pitch. The servo bursts are precisely offset to either side of a data track's center line (112 and 114) as shown in FIG. 1a. The track pitch comprises the write element width, which determines the width of the data track, plus the desired radial distance or separation between adjacent data tracks. The standard method of servowriting these patterns comprises two half tracks servo burst writes performed on separate revolutions which are joined together with a seam. As the width of the product write head is wider than the servo track pitch, each write results in a half track write, which is greater than the servo track pitch. As a result, the second half of the servo burst overhangs the track boundary. In a subsequent step the servoburst is trimmed such that the servoburst width is equal to twice the servo track pitch. This trimming step in which the write head trims the A burst, and writes the first half of the B and the second half of the D burst, occurs in the same revolution.
A commonly used servopattern, known as quad burst is shown in FIG. 1A. The servowrite process to write the quad burst pattern is described with respect to FIGS. 1B and 1C. As shown in FIG. 1B, with the recording write head on servo track 1 an A burst 101 and C burst 102 are written. The burst radial length is determined by the width of the write head and is typically wider than a servo track, which in the case described is half of the data track pitch.
The write head is then stepped forward one servo track. On servo track 2, servo bursts A 103 and D 204 are written. Servo burst A 103 is a second burst in slot A and overlaps a first burst in slot A 101 on servo track 1.
The write head is stepped forward to servo track 3. As shown in FIG. 1C, the write head trims the second burst in slot A 103 such that the A bursts, 101 and 103, are equal to two servotracks or one datatrack. Immediately after the trim operation, burst 105 is written in slot B and a second burst 106 is written in slot D, overlapping a first burst 104. The step of trimming burst 103 and writing burst 105 results in a correlation of an edge of burst 103 and an edge of burst 105 along a boundary formed between servo track 2 and servo track 3. As the location of the data track is determined by the edges of the A and B burst slots for that sector, any error in the position of the write head due to mechanical motion during the writing of that sector are written into the servopattern and become a permanent error that is commonly referred to as repeatable runout term. That is, because the A burst and B burst edges are defined during the same revolution, then non-repeatable mechanical motion of the head during the servowrite, or non-repeatable runout (NRRO), is written directly into the servo pattern track and becomes repeatable runout (RRO).
In disk drive servowriting, the NRRO and therefore written in RRO, is typically uncorrelated between adjacent tracks. The result is an error between adjacent track centers, referred to as AC squeeze, which is the square root of two (2) multiplied by the RRO. AC squeeze is a significant contributor to a track mis-registration budget that ultimately determines the drive track density and is an important limitation in determining the drive track density.
Seamless servo patterns in which servo bursts are comprised of a single write reduce RRO. However, the radial length of the servo burst for a seamless pattern is determined by the write element which varies significantly from the data track width. This variation in servo burst radial length results in large non-linearities of the position signal derived from the servo pattern by taking the normalized amplitudes of the A and B bursts and determining the difference A−B. As each recording medium has a recording transducer with a write element, the servo burst radial length and non-linearity will vary between recording surfaces.
Therefore, a need exists for a method of improving servopattern errors due to non-repeatable runout.